In theory, the power you put into the PRI side of a transformer is that available to you on the SEC side. This statement excludes losses within the core and windings but it is very useful in understanding how transformers work.

Transformers can “transform” voltage, current, and impedance. One type of transformer is known as a step-down transformer. This type of transformer steps down the input voltage to something lower on the SEC side. In the process, it steps up the current because the input power is conserved in theory with the output power. Also, because impedance is the ratio of voltage to current it also steps down the impedance.

It does this through what is known as the turns ratio. Let’s show this using some math and Faraday’s Law of Induction….

This would be the applied PRI voltage. That applied voltage sets up a changing flux within the core, the more PRI turns the lower the rate of change in flux in the core with time. We can write the same equation about the SEC winding.

Since the voltage at the SEC is induced voltage this says that induced voltage is the product of the SEC turns and the rate of change in flux in the core with time.

The PRI and SEC share the same core and so they share the same change in flux in the core with time. We can then relate these two above equations….

Or simply…

The turns ratio is then what relates the SEC voltage to the PRI voltage…this can be seen as…

We also said the power was conserved, so we can write this as:

You’ll notice the SEC current changes in the exact opposite way the SEC voltage does. As the SEC voltage goes down, the SEC current goes up.

Lastly, let’s show how the impedance changes….

As the SEC voltage is stepped down by using fewer SEC turns than PRI turns, the impedance changes with the square of this. This is because impedance is the ratio of voltage to current and not only is the voltage decreasing but the current is increasing in the same exact proportion, so the impedance changes to the second power….

Okay, so this is what we know…..using fewer SEC turns than PRI turns results in a stepped-down voltage at the SEC, a stepped-up current at the SEC, and a stepped-down impedance of the SEC.

This is useful in a situation where the voltage you have available is more than what you desire. For instance, if you have 240VAC available but you require 120VAC then you need to step down the voltage by a factor of 2. So we make the SEC turns be half the PRI turns….let’s do this and see what the equations above show…So…..

Now just as a step-down transformer can be made by using fewer SEC turns than PRI turns a step-up transformer can be made by using more SEC turns than PRI turns. Everything about the step-up transformer is the opposite of the step-down transformer. Voltage goes up, current goes down and impedance goes up squared.